Much of our current genetic understanding of early embryonic development is derived from screens designed to identify zygotic mutations. It is becoming increasingly evident that much of early development is independent of the zygotic genome. Maternal supplies of mRNA and protein are critical for DNA replication, cell division, and patterning during early embryogenesis. The mid-blastula transition (MBT) is the interface between the maternal and zygotic developmental program, and it requires maternal genes for its proper execution. At the MBT, the rapid cell divisions of the cleavage period slow, and a major increase in zygotic gene expression ensues. This proposal will help elucidate the mechanisms involved in the progression of the MBT in vertebrates. It seeks to characterize a recently identified maternal-effect mutant, screeching halt (srh), which arrests development at the MBT in zebrafish. The proposed experiments will investigate the role of srh in checkpoint regulation, a process important to the MBT in both Drosophila and Xenopus. I will determine the molecular identity of the srh gene, which is likely a novel factor acting in the MBT. In addition, I will perform a maternal-effect mutagenesis screen to identify additional alleles of srh, as well as novel genes that also function at the MBT.